It is already known that, by switching between the filaments, an "H4" type lamp or the like can be used in a single headlight to form either a main beam or an anti-fog beam. In this respect, it is recalled that the design purpose of an H4 lamp is to enable a headlight fitted with such a lamp disposed in association with a parabolic type reflector to produce either a main beam or else a dipped beam that is delimited by a "European" cutoff whose outline is in the form of a flattened V-shape. To this end, the H4 lamp includes two axial filaments, with the front one of the filaments being provided with an occulting cup. The filament associated with the cup co-operates with a parabolic reflector to form a dipped beam whose cutoff profile is determined by the geometry of the cup. The disposition of the front filament F and of the cup C is shown in FIG. 1a of the drawings. The angle .alpha. determines the angle at which the sloping portion of the cutoff rises, as shown in FIG. 2a.
Conventionally, a main beam and anti-fog beam headlight of this type includes the above-mentioned "H4 " lamp, a reflector, and a glass. In this case, the lamp is not disposed in the manner intended for obtaining a main/dipped beam headlight, but is angularly offset by .alpha./2 for reasons of symmetry that are explained below. The position of the front filament F and of the cup C is illustrated in FIG. 1b.
In a first known embodiment, the reflector is constituted by two half-paraboloids with the boundary between them being constituted by a horizontal plane that includes the optical axis, the two half-paraboloids being focused respectively on the filament associated with a cup and on the filament without a cup so as to form the anti-fog beam and the main beam, respectively.
A drawback of that known headlight lies in the existence of a step in the reflector between its upper and lower surfaces, said step giving rise firstly to optical anomalies in the resulting beams and secondly to difficulties to manufacturing the reflector, regardless of whether manufacture is by stamping metal sheet or by molding plastics material, with these difficulties including difficulties of varnishing the reflecting surface.
Another known solution for making the reflector consists in using a single paraboloid which is focused on the filament that is associated with the cup. This solution may indeed mitigate the above-mentioned drawbacks, but in order to obtain a main beam with adequate concentration on the optical axis, it becomes necessary to fit the reflector with a system of prisms for ensuring proper refocusing of the main beam.
Finally, and above all, a drawback which is common to both known solutions lies in it being impossible to obtain an anti-fog beam with a horizontal cutoff as is generally desired. More precisely, the very design of the occulting cup which extends over an angle of only 165.degree. around the corresponding filament causes the resulting anti-fog beam to have a symmetrical V-shaped cutoff, with a large quantity of light escaping over the horizontal. The general shape of the resulting beam is shown in FIG. 2b.